Summary

Introduce the CollectionAcquirer seam used by the change-stream single-document lookup
executors to decouple "give me a CollectionAcquisition" from "take a lock". Ships both
implementations and the point-in-time read guard. No pipeline wiring yet – this is a building
block consumed by the Express executor (later in this milestone) and reused later by SBE and
$search.

Scope

• Define the CollectionAcquirer abstract interface returning a wrapped
  CollectionAcquisition handle.
• OnDemandCollectionAcquirer: calls acquireCollectionMaybeLockFree per call. The
  acquirer the Express path uses (target nss varies per event on db/cluster streams).
• PreAcquiredCollectionAcquirer: locates the upfront AcquiredCollection in
  TransactionResources::acquiredCollections and wraps it with no new lock. {{tassert}}s when
  the requested nss is absent from the upfront list. Propagates exists() == false as the
  authoritative answer at the stream's clusterTime. Ships as the reusable building block for
  $search / SBE; not wired into the pipeline here.
• assertLocalLookupReadAtOrAfter() snapshot guard: reads the RU point-in-time read timestamp
  and tasserts it is at or after the event's afterClusterTime. No-op for an absent or
  untimestamped read.

Files

• src/mongo/db/exec/agg/single_doc_lookup/collection_acquirer.h (header-only)
• src/mongo/db/exec/agg/single_doc_lookup/BUILD.bazel

Acceptance criteria

• Both acquirers implemented behind the common interface; PreAcquired tasserts on a missing nss
  and surfaces exists() == false unchanged.
• assertLocalLookupReadAtOrAfter() is a no-op for an absent/untimestamped read and tasserts
  otherwise.
• Unit tests cover: OnDemand acquire, PreAcquired hit / miss-tassert / non-existent-collection,
  and the snapshot guard.